Refactor surface damaging and rendering code

This is based on code from Sway, which is also MIT licensed hence
compatible. This makes the surface damaging and rendering code easier to
follow and makes it easier to import future changes to Sway as well.

util.h

@@ -0,0 +1,11 @@
+#ifndef CG_UTIL_H
+#define CG_UTIL_H
+
+#include <wlr/types/wlr_box.h>
+
+/** Apply scale to a width or height. */
+int scale_length(int length, int offset, float scale);
+
+void scale_box(struct wlr_box *box, float scale);
+
+#endif